One evening in December 1989, John Gotti had a conversation with his acting underboss, Frank Locascio, about murdering a troublesome underling in the Gambino crime family who had failed to show up to a meeting.

Mr. Locascio predicted Mr. DiBono would be bringing Mr. Gotti a large sum of cash the next day. “But I wouldn’t take nothing,” the crime boss answered, affirming in colorful language that Mr. DiBono’s days were numbered.

That taped conversation, along with the testimony of the family’s third-in-command, Salvatore (Sammy the Bull) Gravano, was enough to convict Mr. Locascio of conspiring to murder Mr. DiBono, who met his end violently in a parking garage 10 months later. Mr. Locascio received a life sentence for murder and racketeering.

For years, Mr. Locascio, who is 83, has insisted that he tried to talk Mr. Gotti out of killing Mr. DiBono. He claims he tried to broker a deal that Mr. DiBono would pay Mr. Gotti $50,000 to make peace.

And he contends the same audiotapes used to convict him, if enhanced with modern digital techniques, would prove he is right. Many of his words are inaudible during the conversation with Mr. Gotti, swallowed up in music and background noise.

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Frank Locascio

Having exhausted his other avenues of appeal, Mr. Locascio is now locked in a legal battle with the federal government over access to two of the tapes, which were sealed by Judge I. Leo Glasser in 1998. He has filed a Freedom of Information Act lawsuit in Washington seeking to force the Federal Bureau of Investigation to hand over the tapes so his audio expert may analyze them with digital tools.

He faces more than legal obstacles. Justice Department lawyers said in February that the original seven inch reel-to-reel recordings made by F.B.I. agents had been damaged beyond repair when Hurricane Sandy flooded the agency’s storage facility in New Jersey. In court papers, the Justice Department has also said officials searched the 151 boxes of records from the trial and could not find copies of the two tapes Mr. Locascio is seeking.

Even if undamaged copies of the recordings are located, the Justice Department contends it does not have to release them under the FOIA, according to court papers. Though the tapes were played in court and the transcripts have been published in a book, they are now under seal and are no longer in the public domain, the government argued in court filings.

Complicating matters, F.B.I. officials in Washington handed over a redacted copy of one of the tapes on a compact disc in late March, only to ask for it back a few days later, saying they had made a mistake. On Monday, prosecutors went into Federal District Court in Brooklyn and asked Judge Glasser to order Mr. Locascio’s lawyers to return the CD.

The suit is the latest in a long series of unsuccessful attempts by Mr. Locascio’s lawyers to get a new trial. Judge Glasser and the United States Court of Appeals for the Second Circuit have repeatedly upheld his conviction — five times.

In 2011, Judge Glasser also rejected Mr. Locascio’s request to unseal the tapes, noting that his lawyers were given copies of the original recordings at trial. The judge said he had ruled against Mr. Locascio at trial when he tried to call on an audio expert to interpret the tapes for the jury. That ruling still stood, he wrote.

“This is a thinly disguised effort to re-litigate a ruling made during the trial upon which Locascio and his counsel can only be described as fixated,” he wrote.

Mr. Locascio’s lawyer, Ruth M. Liebesman, said Mr. Locascio should be able to analyze the original tapes with modern audio techniques, just as other defendants have been allowed to re-examine old DNA evidence with advanced technology. “Why shouldn’t we be able to do that with tapes as well?” she said.

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John Gotti in 1990 after being acquitted of assault.CreditSteve Berman/The New York Times

The recordings were made with a bug placed in an older woman’s apartment above the Ravenite Social Club on Mulberry Street in Little Italy in Manhattan, where Mr. Gotti held court. The jury never heard the original reel-to-reel recordings. Copies were made on cassettes, and the originals were put back in the F.B.I.’s safe at 26 Federal Plaza in Manhattan, an agent, George Gabriel, testified.

The agent also testified the tapes had not been enhanced or altered before the jury heard them, a contention Mr. Locascio’s audio expert disputes.

Even if Mr. Locascio obtains the original tapes and can tease out a clearer recording of his words by digitally filtering out distortion and background noise, he is unlikely to persuade the courts to grant him a new trial. For starters, Mr. Gravano testified that no murders were committed without the agreement of both Mr. Gotti and Mr. Locascio.

The jury also seemed to be persuaded by an F.B.I. agent’s testimony that the December 1989 conversation between Mr. Gotti and Mr. Locascio showed the decision to kill Mr. DiBono had already been made, and no payment he might make would change the plan. Judge Glasser highlighted this point in a 2005 opinion denying Mr. Locascio a new trial.

In a 2011 affidavit supporting his request to unseal the tapes, Mr. Locascio remembered things differently. “It is my clear recollection that, after telling Gotti that ‘I predict that he’s going to bring you fifty,’ I told Gotti, in sum and substance, to take the money and forget about his anger with DiBono,” he said, adding: “I had nothing to do with the DiBono’s murder.”

For Mr. Locascio, time is growing short. Had he been convicted only of racketeering, the maximum sentence would have been 20 years and he would now be free. But the murder conviction means he will die in prison, just as John Gotti did in 2002.

Mr. Locascio is incarcerated in a prison hospital, the Federal Medical Center Devens in Ayer, Mass. To pay legal bills, his family has sold an upstate horse farm he owned. He uses a wheelchair, has emphysema and cannot stand on his own. Most days, Mr. Locascio maintains an old street tough’s optimism about his chances of release, despite the long odds, his nephew, also named Frank Locascio, said. Other days are darker.

“I only heard him down in one visit,” his nephew said. “He was saying ‘I’m the little fish in the fish tank and the water’s evaporating.’”

A version of this article appears in print on , on Page A17 of the New York edition with the headline: Gotti Underboss Fights for Audiotapes He Says Prove He Didn’t Plot Murder. Order Reprints | Today’s Paper | Subscribe